1. Field of the Invention
This invention relates to display devices utilizing an electrochemical coloration-bleaching phenomenon, that is, an electrochromic (EC) phenomenon and, more particularly, all solid state EC display devices using a reduction coloration type EC material layer and an oxidation coloration type EC material layer.
2. Description of the Prior Art
An all solid state complementary electrochromic display device, for example, disclosed in Japanese Published unexamined Patent Application No. 56-4679 issued by the Japanese Patent Office on Jan. 19, 1981, includes a pair of electrode layers, an oxidation coloration type EC material layer which is capable of oxidation and reduction reactions and is colored in an oxidation state, a reduction coloration type EC material layer which is capable of oxidation and reduction reactions and is colored in a reduction state, and an ion permeable insulating layer which is interposed between both the EC material layers and passes protons but blocks electrons, these layers being disposed between said pair of electrode layers. With this arrangement, by the application of a voltage across the electrodes, the EC material layers exhibit the coloration-bleaching phenomenon simultaneously. Compared with a conventional EC display device using a single EC material of either an oxidation or a reduction coloration type, this complementary EC structure has an improved response time; the time required until the optical density reaches 1.0 is 0.5 seconds while it has been several seconds in the past. However, in order to display the seconds of a watch, the response time should be reduced to about 100 ms. Thus, the response time is not satisfactory, resulting in the low reliability of the device.
For another electrochromic display device, a U.S. Pat. was issued on Apr. 8, 1979 (see U.S. Pat. No. 4,147,416). This device includes "a transparent front substrate, at least one transparent display electrode provided on the front substrate, a counter substrate provided parallel and adjacent the front substrate, at least one display electrode provided on the counter substrate adjacent the display electrode on the front substrate, a transparent electrolyte enclosed within the substrate, an electrochromic material provided in the electrolyte a counter electrode which is coupled to the electrolyte and a means for selectively applying voltage between the counter electrode and the display electrodes" (column 2, lines 3-14). In this device, by driving both the electrodes to produce the half of the coloration required for the display, the normal colored stale can be obtained through the overlapping electrodes. This device has a fast response time and makes the second display possible. However, using two substrates results in a total device thickness of about 1.5-3 mm including the electrolyte. Further, for the fabrication of the device, some processes of injecting the electrolyte, bonding or the like are necessary.